This invention relates to copper arsenate slurry compositions which are useful in preparing chromated copper arsenate wood preserving formulations. More particularly, it relates to copper arsenate slurries having a molar ratio of As.sub.2 O.sub.5 to CuO of from about 0.3:1 to about 0.9:1 and which are readily soluble in chromic acid. This invention also relates to a method for preparing the copper arsenate slurries from copper metal and arsenic acid.
During recent years the group of wood preservatives known as chromated copper arsenates (CCA compounds) have become of great importance. These CCA compositions are one of the most widely used type of wood preserving compounds in use today. These preservatives contain as active ingredients copper as cupric ion, chromium usually as the dichromate ion and arsenic as acid arsenate ion. In addition, the CCA compositions may also contain inert ions and salts such as sodium ion, sulfate ion, and/or sodium sulfate. These inert ions and salts are the side products which are formed when the CCA compositions are formulated from the least expensive source of active ingredients usually available; for example, copper sulfate as the copper source and sodium dichromate as the chromium source.
Typical formulations of some of the widely used CCA compositions of the prior art are as follows: Greensalt which is 56% Na.sub.2 Cr.sub.2 O.sub.7. 2H.sub.2 O or K.sub.2 Cr.sub.2 O.sub.7, 33% CuSO.sub.4 . 5H.sub.2 O, 11% As.sub.2 O.sub.5 . 2H.sub.2 O; Tanalith C which is 45% Na.sub.2 Cr.sub.2 O.sub.7 . 2H.sub.2 O or K.sub.2 Cr.sub.2 O.sub.7, 35% CuSO.sub.4 . 5H.sub.2 O, 20% As.sub.2 O.sub.5 . 2H.sub.2 O; and Celcure A which is 40% Na.sub.2 Cr.sub.2 O.sub.7 . 2H.sub.2 O or K.sub.2 Cr.sub.2 O.sub.7, 32% CuSO.sub.4 . 5H.sub.2 O, 26% As.sub.2 O.sub.5 . 2H.sub.2 O. In recent years, the American Wood Preservers Association (AWPA) has standardized the formulations of CCA compositions. The formulations have been converted for greater ease in specification into a uniform nomenclature based on the oxides CrO.sub.3, CuO and As.sub.2 O.sub.5. The currently adapted standardized American formulations have the following compositions in percent by weight:
______________________________________ CCA-Type A CCA-Type B CCA-Type C ______________________________________ CrO.sub.3 % 65.5 35.3 47.5 CuO% 18.1 19.6 18.5 As.sub.2 O.sub.5 % 16.4 45.1 34.0 ______________________________________
The formulations are used as solutions containing from about 45 to 75% active oxides in water.
While the CCA formulations have found widespread use in the art, they generally suffer from one or more drawbacks. For example, it is difficult to formulate the CCA compositions as dry powders, especially for use in the southern United States. The CuSO.sub.4 . 5H.sub.2 O readily gives up its water of hydration to the As.sub.2 O.sub.5 . 2H.sub.2 O or Na.sub.2 Cr.sub.2 O.sub.7 . 2H.sub.2 O, forming a very heavy, almost unmanageable paste. Some improvement can be effected by using potassium dichromate instead of the sodium salt, but this compound is substantially more expensive. The responsibility for mixing the formulation is thus transferred in many cases to the individual woodtreating plant, which frequently does not possess the specialized technical equipment needed to prepare accurate working solutions of these toxic and irritating chemicals.
In addition, the presence of the inert salts which are formed make it impractical to ship a concentrated solution from a central distribution to a treating plant since the alkali metal sulfates are the least soluble salts in such a solution and they precipitate out at moderate concentrations. Moreover, the presence of these inert salts have a deleterious effect on some applications of the CCA formulations. As the applications of these preservatives increased, they found more and more use in the treatment of utility poles. For this use, the residual inert salts remaining in the wood after the fixation of the preservative has occurred are undesirable since they have a deleterious effect on the treated utility poles.
Because of the deleterious effect of the inert salts, compositions have been formulated which eliminate these inert salts. For example, see Hager, U.S. Pat. No. 2,432,007 Dec. 2, 1947) and McMahon, U.S. Pat. No. 2,438,511 (Mar. 30, 1948).
Ideally, the CCA formulations should be aqueous solutions free from alkali salts containing about 60% of active oxides, which are inexpensive to ship and convenient to use in the treating plants. Such concentrated solutions have been prepared and are currently available. They are readily prepared from copper hydroxide or basic copper carbonate, chromic acid (chromium trioxide) and arsenic acid. Unfortunately, copper hydroxide and copper carbonate are much more expensive than copper sulfate so that advantages and conveniences of a liquid concentrate free from inert salts are largely offset by increased costs.
Attempts have been made to circumvent this difficulty in the case of CCA-Type B, which has the highest arsenic acid content of any of the standard CCA formulations. In U.S. Pat. No. 2,565,175, Hager discloses the manufacture of a copper arsenate from copper sulfate and arsenic acid. This mixture is treated with a carefully measured amount of calcium hydroxide, which precipitates most of the sulfate as gypsum (CaSO.sub.4 . 2H.sub.2 O). In order to avoid loss of arsenate through precipitation, a deficiency of calcium hydroxide is used, so the final copper arsenate may contain about 3% or so of the sulfate. This process for the manufacture of copper arsenate as an intermediate for the manufacture of chromated copper arsenate has several drawbacks.
First, this process is limited to the production of a copper arsenate in which the mole ratio of As.sub.2 O.sub.5 to CuO is about 1:1, yielding a completely water soluble copper arsenate. These high mole ratios are typical of Type B CCA formulations and cannot be used for the manufacture of Type A or Type C products where the As.sub.2 O.sub.5 :CuO ratio is much lower. In addition, as in well known in the art, precipitated gypsum is a material which is very voluminous in nature and extremely difficult to filter and wash. Much of the economy effected by the use of the relatively inexpensive copper sulfate is lost through the expense of handling the precipitated gypsum.
It is, therefore, an object of this invention to provide a concentrated aqueous CCA formulation which does not contain any appreciable soluble inert ions or salts, which is inexpensive to make, convenient to use, and inexpensive to ship. It is also an object of this invention to provide copper arsenate compositions which may be mixed with chromic acid to form the CCA formulations having the properties described above. The copper arsenates of this invention are inexpensive, easy to prepare, react readily with chromic acid, contain a minimum amount of inert ingredients and are capable of being used to make all of the various CCA formulations. In addition, this invention has as an object a method of preparing the copper arsenate compositions from inexpensive and readily available starting materials.